"It doesn't stand much chance of being produced, but it especially doesn't stand a chance so long as the main character's predilection for child pornography is a central part of the story. If it has to be involved, it's because the oldest two girls put on shows on some sort of video chat site - which the man catches them at."

Not implausible. I saw a news article complaining, inter alia, about a 7 year old girl making a video of pleasuring herself, and putting it on Youtube without anyone´s assistance or prompting.

But that could undermine the sympathy of the audience with the girl.

On the other hand... say, he likes to see skimpily dressed children, so looks at catalogues of children´s underwear and bathsuits, on pretext of looking for some that would fit his girl. The girl figures out that he´d like to see her... so insists on trying on the underwear in front of him. "Aren´t you ashamed to be seen naked? Don´t you mind?" - "It´s you! I like you. I would mind even so, if you weren´t asking me. Since I´m offering, there is nothing to be ashamed of!"